The cutting efficiency of an instrument depends in large part on the angle between the long axis of the cutting edge and the direction of the cutting stroke, the blade-stroke angle. An oblique, or slicing, angle is more efficient than a perpendicular, chopping one. Oblique cutting has been used from the early stone knife and flat arrowhead to the scythe, saber and scimitar, sawtooth, scissors and even the guillotine. The axe and modern safety razor are the only common tools that use a perpendicular angle and chop, rather than slice. In fact, the axe is often made with a curved blade or used with a slicing stroke, and some prehistoric, bronze age razors had curved blades so as to shave obliquely. The modern, conventional wet safety razor, however, still uses only a perpendicular angle and cuts by hacking, i.e. perpendicular chopping.
Compared to perpendicular chopping, oblique slicing cuts more smoothly and the cutting edge remains sharp longer. Oblique razors have not been successful, however, despite many attempts, as evidenced by more than twenty United States patents granted from 1880 through 1992. The inventions failed for several reasons.
Razors that shaved only obliquely were unsuitable to shave near the sideburns, lips or nose, where perpendicular shaving is easier and safer. Oblique razors that also shaved perpendicularly were costly and cumbersome. Further, these razors generally did not ensure an optimum oblique angle; most were rotary, allowing angles over a 360.degree. range. Oblique angles outside a narrow range are undesirable. Small angles are not worthwhile; large ones excessively reduce the area shaved by each stroke. Further, a widely angled blade passes too deeply over raised surfaces and cuts skin with the same efficiency as it cuts hair, while it skips depressed surfaces. Excessive obliquity cuts the skin, also, because the effective distance of the blade guard from the following blade is significantly increased. Dumas (U.S. Pat. No. 4,791,724) and Gorden (U.S. Pat. No. 3,964,160) patented razors that shaved perpendicularly and at safer oblique angles, but both needed special razor heads, and Gorden allowed ineffective oblique angles. Dumas's razor shaved perpendicularly and was incidentally limited to one oblique angle by the "V" arrangement of its unusual cartridges.
All razors that shaved both obliquely and perpendicularly were inconvenient to convert back and forth, requiring that shaving be interrupted to change the angle. The fingers or the hand had to be displaced, or both hands were needed. Althaus' razor (U.S. Pat. No. 5,033,152) had a spring to return its blade unit automatically to a single neutral position, but the razor had no specific oblique angle. The spring merely allowed the blade to be passively deflected by chance skin surface irregularities; the razor shaved obliquely by accident and under no control.
Every previous razor had at least one major defect. A practical razor for oblique shaving must not have any. The razor must shave obliquely at an effective angle. It must not permit oblique shaving outside a limited, optimum range. It must shave perpendicularly when desired. The shift of shaving angles must be convenient; it must not interrupt shaving or require a second hand or displacement of the fingers from their normal hold on the razor. The shift in angles must be safe. The razor must be economical, preferably having a conventional blade unit, which should last longer with oblique shaving. Each of these requirements is essential.